In the first place I would like to beg for your forgiveness for my late response. Your mirror had arrived Hong Kong early this year and WongSir demanded that the mirror must be handled with great care, avoiding upsetting your great support.

You had mentioned about using the auto-collimation test for figuring this mirror. This test is well-known for its extreme precision and a mirror, under such superb test, could easily reach an error not exceeding 1/20 wave P.V. I firmly believed that what I could do was just to reconfirm your result.

Due to the lack of a sizeable standard optical flat, I decided to test the mirror by the elementary Foucalt knife-edge test. Though primitive the method was, great precautions were engaged in order to minimize the errors as mush as possible. These included the use of a big laboratory with all its windows closed aiming to stabilize its environmental temperature. All data were sampled only after a long wait until not a single trace of air turbulence was observed over the mirror. Finally all single data was an average of three samples to make sure for reliable results. Nevertheless the results obtained was quite departed from yours, even under numerous tests, day after day, experiment after experiment. Basically the profile of your mirror is as follows :-

1. There is a shallow “depression”, diameter 4 to 5 cm , lying at center of the mirror. This is by no means surprising since the central portion of the mirror was blocked during the auto-collimation test. There was just was no way to tell how good or how bad the mirror was at this region.

2. Ignoring central portion, the mirror is basically a good paraboloid,. However test after test the results obtained only indicated a 1/6 to 1/8 wave P.V. accuracy. A 1/20 wave P.V. result was never achieved no matter how many times the experiment was carried out.

I belief that I am the one who brought about all these troubles. It is a well known fact that the Foucalt test could accomplish good precision only when the mirror under test is of sufficient long focal length. For fast mirrors, upon knife edge cutting, shadow transient was too quick for average human eye to capture equal grayness accurately. Moreover observer’s subjective judgement also leads to rapid growth of random errors. Mr. So’s mirror is a f/5. With such short focal length together with my poor eyesight it was not surprising that the test was not at all carried out competently.

Now we have a situation here which would, regretfully, lead to the disappointment of both Mr. So and WongSir. In order to fulfill my mission I made my decisive move to make another mirror with accuracy also of 1/20 wave P.V. and hopefully this reference mirror could be AB tested against Mr.So’s, giving a more sounding result. To me who equips with only mediocre ATMing technique the making of a mirror of such precision means a colossal job taking great length of time. It is not until recently that the reference mirror has been finished and with an error envelop of, despite my greatest effort spent, not better than 1/16 wave P.V. It deems that I am a worse ATMer comparing with Mr. So.

To truly fabricate an AB test we need to take care of a lot of recourses – such as two mirror testing rigs, two equatorial mounts and two pieces of identical diagonal flats from the same factory with equal dimensions. The list could be exhausting and preparation job is still not fully ready at the moment. However the individual star test of each mirror has been carried out. Under a magnification of 500X both mirrors performed very nearly identical. If we have to make the preliminary judgement between them all we can say at the moment is that the reference mirror gave a little more sharper and detailed image while the contrast of Mr. So’s mirror was a bit higher. Nevertheless during these two independent observations atmospheric turbulence were not exactly the same and the result was believed to be far from final. At the moment we are still waiting for arrival of all required equipment and hopefully we will come to a decisive conclusion shortly.

Nowadays commercially available astronomical telescopes selling at low prices are pouring into the market and consequently ATMing is a declining hobby both in its quantity and quality. ATMers with superb technique is rarely seen and Mr. So is definitely one of those. I am being inspired, and honored, to have a chance to see your excellent mirror and it is my hope that we will still enjoy the luck to have your continual offering of opinion to improve our ATMing technique.

Unfoertunately we don't have sufficient equipment to make such pictures. There are difficulties :-

1. The therorectical Airy disc diameter of and 8 inch mirror is about 0.6 arc sec which demands very steady atmosphere for shooting, in order to see the disc and surronding rings.

2. After shooting the picture must go through some image processing which, unfortunately, cover up the minute difference between the two mirrors ( we are talking about a difference of 1/16 wave to 1/20 wave!!! )

3. Finally, and most importantly, we don't have such a good equatorial mount that have motor vibration smaller than 0.5 arc sec ( Even a Takahashi EM-1, EM-1s cannot do it, I don't know whether a EM-11, EM-200 could do it but we don't have two such units identical ) making comparision impossible. All previous star tests were carried out by using hand slow motion tracking to avoid motor vibration. Not shooting is possible undersuch arrangment.

We do hope that a true AB star test, visually, could be carried out shortly, to arrive at final conclusion.